Many conventional drug delivery devices are provided to a patient without a liquid drug prefilled in a drug container of the device. The patient is therefore often required to fill the drug device prior to use. These conventional drug delivery devices can typically include unreliable complex systems for expelling the liquid drug from the drug device, thereby increasing the costs to the user. Further, these conventional drug delivery devices often do not allow a variable amount (or user-selectable amount) of liquid drug to be inserted into the drug container so that the patient can vary dosages. Accordingly, there is a need for an improved drug delivery device that allows for variable filling and expels the stored liquid drug in a less costly but more reliable manner.